Researchers studying the pathophysiology of diabetes mellitus identify a family with an inherited mutation in the insulin gene. The mutation results in a single amino acid substitution in the proinsulin molecule. However, affected family members are asymptomatic, and most have completely normal blood glucose levels. Analysis of the mature insulin molecule shows that it is identical to normal human insulin. The researchers conclude that the amino acid substitution affects a segment of proinsulin removed during post-translational processing of the protein. The polypeptide segment containing the mutation is most likely to undergo which of the following processes within pancreatic beta cells?
Show Explanatory Sources
Insulin, a polypeptide hormone synthesized by pancreatic beta cells, is released in response to elevated glucose levels. Insulin mRNA codes for preproinsulin, a larger precursor molecule that begins with a hydrophobic N-terminal signal peptide. Shortly after translation begins, this signal sequence interacts with cytosolic signal recognition peptide, which halts protein synthesis until the ribosome translocates to the rough endoplasmic reticulum (RER). Once bound to the RER, translation resumes and the growing peptide chain is extruded into the RER. The N-terminal signal peptide is then cleaved from preproinsulin to yield proinsulin.
Proinsulin undergoes conformational changes and the addition of 3 disulfide bonds in the RER. Proinsulin is then transported to the Golgi apparatus, where it is packed into secretory granules. Endopeptidases in the secretory granules cleave proinsulin into insulin and C-peptide, which are then stored within the granules until they are secreted from the cell via exocytosis (Choices A, B, and C).
Exogenous insulin administered to patients with diabetes does not contain C-peptide. However, C-peptide released from beta cells remains in the circulation with a half-life of approximately 35 minutes and can be assayed as a surrogate marker for endogenous insulin production.
(Choice D) Most proteins destined for the extracellular space have a signaling sequence that directs them to the RER/Golgi apparatus, where they are packaged into secretory vesicles. However, a few proteins, such as interleukin-1, lack a signaling sequence and are secreted directly from the cytoplasm via an unconventional mechanism.
Educational objective:
Cleavage of proinsulin in pancreatic beta-cell secretory granules yields insulin and C-peptide, which are stored in the granules until they are secreted in equimolar amounts.